Water is the mother solvent for a wide variety of naturally occurring inorganic species. Excessive amounts of such species can render water either unusable for general consumption or specific industrial and agricultural uses or pose a high risk to the environment. The term "excessive", however, depends on the acceptable daily intake or virtual safe concentration for these species. It is therefore imperative to precipitate (concentrate) and separate such species to protect both human health and environment.
In most surface waters (e.g.; seawaters) and subsurface waters (e.g.; oil-field brine waters) the dominant anion is chloride with the exception of few cases where bicarbonate and sulfate exceed chloride by weight. In chloride-rich waters, particularly in seawaters, sodium is the dominant cation. In some subsurface waters, however, chloride-rich waters are divided into two major subtypes: (1) sodium is greatly dominated over calcium; and (2) calcium is relatively abundant. Chloride-rich waters that are high in calcium are generally high in alkaline earth elements such as barium chloride, strontium chloride and radium chloride. The availability of radium in chloride-rich waters suggests that the decay chain of radium or the so-called Naturally Occurring Radioactive Materials (NORM) are common, and thus such waters can become radioactive. Radium isotopes, however, are responsible for more than 90% of the total radioactivity found in subsurface waters.
Naturally occurring selenium in both soil and groundwater, is for the most part, present in trace concentrations ranging from insignificant to as much as 3.7 ppm. Commercially, however, selenium is a by-product of several industries such as copper refinery slimes, dusts from copper and lead smelters, sulfuric acid plant sludge, and others. Industrial uses for selenium include, for instance, photoelectric cell production, ceramics, rubber, glass pigmentation, xerography, and flame-proofing of electric cable. The estimated annual release of selenium to the environment is about 10 million pounds. The narrow difference between nutritionally essential and toxic concentrations endows selenium with a unique interest.
Nitrate (or nitrite) and phosphate salts are not generally found in appreciable amounts in surface or subsurface waters. However, the nitrate nitrogen (NO.sub.3 -N), for instance, is occasionally found in groundwater supplies at concentrations significantly above the recommended limit (10 mg/L). The major sources of this pollution are: (1) nitrified percolation from septic-tank tile fields; (2) drainage and infiltration of fertilizer and feed-lot nitrogen; and (3) groundwater recharge operations using high-rate infiltration of secondary sewage effluents. The nitrate problem in water supplies is widely spread in rural, suburban and even urban areas.
Conventional water treatment processes such as coagulation, filtration (including membrane processes), and chlorination have little effect on the nitrate concentration. This is attributed to: (1) the high solubility of nitrate salts in water; and (2) the lack of co-precipitation and adsorption of nitrate anion. It appears that ion exchange technology is the most promising treatment method for nitrate removal. However, the disposal of the spent nitrate-containing, regenerate-brine solution is an unsolved problem.
As tightening environmental regulations continue to evolve, the disposal problems associate with many separation technologies (e.g., ion exchange and reverse osmosis) are of special interest. Industrial applications such as the electrical and nuclear power industries, for instance, rely heavily on ion exchange as an ultra-pure water technology. Apart from strictly water treatment applications, ion exchange has been used for the concentration, recovery, and purification of valuable inorganic species. The use of membrane processes for the treatment and concentration of cooling tower blowdown of power plants is another example. Each of the above technologies produces brine streams requiring further treatment before disposing (e.g., ponding or downhole injection). Some of the brine streams, however, are contaminated with toxic or radioactive inorganic species.
A treatment for membrane processes in some cases is required to reduce a concentration of scale salts in the feed stream, for instance, to the concentration less than the scale salts saturation aqueous solubilities in the brine stream (30% of the feed stream). Such a treatment is necessitated by the fact that precipitation of scale salts could quickly impede the rejection capability of the membrane and cause membrane fouling.
Identification of a novel process to concentrate and separate inorganic species from aqueous solutions is thus urgently needed. The identified process can either stand alone on its own merits or serve as an integral part of hybrid system in conjunction with other separation technologies. Optimum process development, design, and operation could make disposal alternatives such as downhole injection a more feasible option.
The above situations have led me to invent the process that is described in this invention to alleviate the problems associated with the existence of inorganic species in aqueous solutions.